Generally, in most cases, power line communication uses a method of modulating a signal (a communication signal) desired to communicate and overlapping the modulated signal on a power line to which a corresponding device is connected.
However, a communication signal modulated by the power line communication method acts as a noise in a general device which is connected to the same power system and does not desire communication.
In addition, when a certain device connected to the power system generates a lot of noises, a communication error occurs, and thus the communication speed is lowered. In addition, the modulated signal generally has a high frequency, and if a peripheral device having a condenser at an input unit exists in the power system, impedance is lowered greatly for a corresponding modulated signal, and this will almost short-circuits the device, and the signal cannot be transferred to a far distance.
Accordingly, a prior art related to the power line communication has been proposed by the inventor of the present invention to solve the problem.
The power line communication method is a communication method of a new type completely different from an existing power line communication, and this is particularly a technique appropriate to an illumination control field or the like having a small amount of data to be transferred without the need of high transmission speed.
This will be described with reference to companying drawings.
As shown in FIG. 1, a conventional closed-circuit power line communication system includes a transmission means 10 connected to one side of the power line P in series to generate a communication signal and transmit the communication signal through the power line and a plurality of reception and load means 20 connected to both sides of the power line P in parallel to receive the communication signal transmitted through the power line and control a corresponding load.
The transmission means 10 includes at least a control signal generation unit 12 for generating a control signal for generating a power line communication signal, and a switching device 13 connected to one side of the power line in series to perform a switching operation by the control signal generation unit 12.
This will be described in detail.
The control signal generation unit 12 outputs a predetermined switching signal to the switching device 13.
That is, in synchronization with the waveform of a (AC) power flowing through the switching device 13 as shown in FIG. 2(a), the control signal generation unit 12 outputs a switching signal for turning off the switching device 13 for a short time period right after the zero cross of the power waveform as shown in FIG. 2(b).
The switching signal of a form as shown in FIG. 2(b) has a meaning of a communication signal (data), which will be described below.
Accordingly, it is understood that output is blocked for a short time period tΔ right after the zero cross as shown in FIG. 2(c) in the case of a power waveform output through the switching device 13.
Here, it is assumed that if the power waveform is in an OFF state for a predetermined time period tΔ before a next half cycle starts right after a zero cross, this is defined as ‘1’, and if the power waveform is immediately turned to ON at the moment of starting the next half cycle right after the zero cross, this is defined as ‘0’.
Accordingly, FIG. 2(b) shows a switching signal having information ‘1011’, and due to this switching signal, a power waveform transformed as shown in FIG. 2(c) based on the switching signal is simultaneously applied to the plurality of reception and load means 20 through the power line.
Here, the communication information ‘1011’ is an example and can be transformed diversely, and contents of the communication information include information on an ID of a corresponding illumination lamp, as well as control information for turning on and off and dimming the illumination lamp, which is a corresponding load.
In addition, although the transformed power waveform including the communication information has a period where power is turned off for a time period tΔ as shown in FIG. 2(c), this time period tΔ is extremely short and does not fail to supply a rated power as a whole.
A key input unit 11 of the transmission means 10 is a switch circuit for inputting a key signal to generate and transfer a control signal, i.e., a switching signal, having information ‘1011’ to the control signal generation unit 12.
Meanwhile, the reception and load means 20 is a plurality of load units connected to the closed-circuit power line P in parallel and includes a power supply unit 21, a load control unit 22, an illumination lamp 23, a signal detection unit 24 and a signal processing unit 25.
The power supply unit 21 receives (AC) power input from the power line P and supplies a driving power to the illumination lamp 23 and a DC driving power to each circuit unit.
The load control unit 22 receives and analyzes the communication signal received from the signal processing unit 25 and controls turning on and off or dimming a corresponding illumination lamp 23 according to the analysis.
The signal detection unit 24 detects the communication signal input through the power line, and the signal processing unit 25 processes the communication signal input from the signal detection unit 24 and inputs the processed communication signal into the load control unit 22.
Operation of the reception and load means 20 configured as such will be described.
A transformed power waveform having information ‘1011’ as shown in FIG. 2(c) is input into the power supply unit 21 and the signal detection unit 24 by the transmission means 10 through the power line P.
The transformed power waveform flowing into the signal detection unit 24 is applied to a light emitting element 24a of a photocoupler PC.
Accordingly, the light emitting element 24a is turned off at a time point tΔ where the power waveform applied as shown in FIG. 2(c) arrives around the zero cross or power is cut off and is turned on at the other part of the power waveform.
A light receiving element 24b also turns on and turns off according to on and off of the light emitting element 24a. 
For reference, since the light emitting element 24a is a device which is turned on only when a minimum rated voltage is supplied, it is turned off due to shortage of voltage around the zero cross point of the power waveform.
A pulse waveform as shown in FIG. 2(d) is output from the output terminal of the light receiving element 24b due to the on and off operation of the light receiving element 24b. 
Output of the photocoupler 24 is processed by the signal processing unit 25 and input into the load control unit 22.
The load control unit 22 detects a value input from the signal processing unit 25, recognizes a pulse P1, P3 or P4 having a predetermined pulse width as signal ‘1’, recognizes a pulse P1 which does not have a predetermined pulse width as signal ‘0’, and determines that the input value is a signal having communication information of ‘1011’.
The load control unit 22 determines whether or not the detected communication information is a signal for controlling its own illumination lamp 23 (confirms an ID), and if it is determined that the communication information is a signal for controlling its own illumination lamp 23, the load control unit outputs a control signal corresponding to the communication signal, i.e., a control signal for turning on and off and/or dimming the light, so that the form of the power provided to the illumination lamp 230 by the power supply unit 21 may be changed.
The power line communication system invented by the inventor provides a lot of advantages such as communication accuracy and the like compared with a conventional power line modulation method.
However, such a power line communication system has problems described below.
First, in creating a communication signal using a power supply, the power cut off period tΔ as shown in FIG. 2 (c) should be long in order to further increase the recognition rate of the communication signal and securely and correctly transmit the communication signal.
However, it is understood that if the power cut off period tΔ is long, voltage Vs is increased at a time point where the switching device 13 of the transmission means 10 is turned on.
As described above, if the voltage Vs at the time point of turning on the switching device 13 is increased, a switching noise is generated.
Second, since the current always flowing through a load flows through the switching device 13 of the transmission means 10, flow of the current is increased, and, particularly, a lot of heat is generated according to the RDSon resistance value when the switching device 13 is turned on.
Due to such phenomena, there is a problem of cost and space since a semiconductor switching device having a large current capacity and a heat sink plate of a large capacity should be used.
In addition, there is a problem in that a lot of electrical energy is lost in the form of heat as the heat is generated.
Third, if a load has a high inrush current due to a large condenser component, there is a problem in that the semiconductor switching device 13 is destroyed since an extremely high inrush current flows when several loads are connected in parallel.